1. Field of the Invention
The present invention relates to a method of manufacturing a magnetic recording medium. More precisely, this invention relates to a method of manufacturing a magnetic recording medium that includes an application process of forming, using a spin coating method, a resist layer on the surface of the magnetic recording medium substrate to perform pattern formation.
2. Description of the Related Art
There has been a trend toward increasing capacity for hard disk drives in recent years, necessitating correspondingly rising recording densities for magnetic recording medium. Together with increased recording densities, increases in the track density have been accompanied by unwanted recording, called side fringes, in regions between adjacent tracks due to a leakage magnetic field occurring from the side faces of the head gap during data writing. These side fringes are a source of noise and cause a decline in the reproduced signal S/N ratio.
In order to avoid such difficulties, proposals have been made relating to a discrete track-type magnetic recording medium, in which grooves are provided between adjacent recording tracks (see, for example, Japanese Patent Application Laid-open No. 2005-108351). In a magnetic recording medium of this discrete track type, by providing grooves between adjacent tracks, the tracks are separated, and so the above-described difficulty of side fringes can be avoided.
On the other hand, in order to attain still higher recording densities, a patterned medium has also been disclosed, in which dots are formed on the disc so as to each become one bit.
The discrete track medium and patterned medium both entail formation of a fine pattern on a disc, and this patterning is often performed using an imprinting method.
In this imprinting method, a mold in molding face on which a fine pattern is formed is used as an original mold, and the fine pattern is transferred onto the surface of a substrate onto which a resist liquid is applied; then, appropriate etching is performed using the fine pattern (resist pattern) transferred onto the resist face, to process the medium surface.
As the method used to apply the resist liquid, a spin coating method is known and is widely used in semiconductor manufacturing.
In the spin coating method, while rotating the substrate rapidly, a photohardening resist, thermoplastic resist, or other resist material in liquid form (hereafter called a resist liquid) is dripped onto the substrate, so that the resist liquid is uniformly spread across and applied to the entire surface of the substrate.
As the magnetic recording medium for large-capacity hard disk drives, normally a disc-shaped magnetic recording medium with magnetic recording performed on both faces is used.
Hence, in manufacturing a magnetic recording medium having a pattern on the surface, it is necessary to process both faces. However, patterns required for a magnetic recording medium have a precision on the order of several tens of nanometers, and preexisting spin coating methods are widely being studied as methods for forming resist layers appropriate for processing with such high precision.
In order to uniformly apply a resin (resist liquid) to a substrate using a spin coating method, the liquid must be dripped onto the center of the substrate, but a center hole is normally formed in the center of the substrate of a magnetic recording medium, and this hole portion impedes the spreading of resin and complicates formation of a film with uniform film thickness. Hence methods are used in which the hole portion at the substrate center is avoided when supplying resin to cause rotation spreading. Further, as a method of supplying resin to one face of the substrate without avoiding the hole portion at the substrate center, methods have been disclosed in which the center portion is covered (see, for example, Japanese Patent Application Laid-open No. 10-320850 and Japanese Patent Application Laid-open No. 11-213459).
In conventional spin coating methods, the rear face of the substrate is held and the resist liquid is applied to the surface to form a resist layer. In the case of a magnetic recording medium, both faces are processed, and so when a pattern is formed on one face at a time, the resist liquid is applied to one face of the disc-shaped substrate having a center hole in the center as described above, and imprinting and etching are performed; but in order to form patterns on both faces, at least twice the manufacturing time is required and, thus, efficiency is greatly reduced.
Further, when processing one face at a time, the rear face is affected during each of the spin coating, imprinting, and etching processes. As a result, media manufacture with the same quality for both faces is difficult.
That is, in forming a resist layer using spin coating, when the resist liquid is applied to the substrate surface, the resist liquid flows around to the rear face of the substrate, and in this state affects the resist application to the rear face, so that there is the problem that uniform application is not possible.
Further, resist which has flowed around to the rear face of the substrate is easily removed from the substrate, become particles which adhere to the substrate, and becoming a cause of defects and unevenness.
Also, when a resist layer is formed on one face at a time, and imprinting and etching processing are performed simultaneously on both faces, the time from application of resist liquid until processing is different for the two faces, so that the amount of solvent remaining in the resist liquid applied to the substrate's surface (front face) and the amount of solvent remaining in the resist liquid applied to the substrate's rear face are different. Consequently, the viscosity of the resist liquid applied is different on the substrate surface and rear face, and if imprinting is performed simultaneously on both faces of the substrate, differences occur in the pattern shapes transferred to the surface and to the rear face, and the transfer is impaired and may be defective.
In methods of covering the center hole portion of a conventional substrate, the resist liquid easily adheres to the cover after resist liquid application and, when the cover is removed from the substrate, the resist liquid easily adheres once again to the unapplied portion on the inside periphery, and moreover flash (unintended excess portions) readily occur where the resist layer is formed by application.
Resist adhering to the peripheral portions of the center hole that were not intended for resist application (unapplied portions) peels away when the substrate is fastened at the inner periphery, becoming particles and causing defects.
On the other hand, if imprinting is performed with flash remaining on the resist layer, not only is uniform imprinting not possible, but pressure is concentrated at the portions in contact and the mold used for imprinting is deformed so that the mold may be degraded.
Also, in methods of the prior art in which the substrate is held by an inner-periphery chuck used in processes to manufacture a magnetic recording medium, the resist liquid flows around the chuck portion and adheres when the resist liquid is applied, so that the difficulty of occurrence of particles during substrate transport occurs.